1. Field of the Invention
The present invention relates to a force-controlled electric hand which drives fingers by an electric motor to grip a target workpiece.
2. Description of the Related Art
As this type of force-controlled electric hand, a robot hand attached to an industrial robot is known. When using a robot hand to grip a target workpiece, it is preferable to make the gripping force easily adjustably, so as to apply a large number of types of shapes of target workpieces to be handled. For example, Japanese Unexamined Patent Publication No. 2011-183513A (JP2011-183513A) describes a gripping device which drives a rotary-linear motion conversion mechanism through a speed reduction mechanism by an electric motor and has a pair of fingers which open and close by the rotary-linear motion conversion mechanism. The device described in JP2011-183513A adjusts the gripping force by adjusting the motor current in accordance with characteristics determined in advance by experiments, etc.
In the device described in JP2011-183513A, friction occurs at the speed reduction mechanism, the rotary-linear motion conversion mechanism, and other drive parts. Therefore, the motor torque is reduced by the friction. Since this friction changes along with the lubricated state of the drive parts, the air temperature, aging, etc., it is difficult to precisely control the gripping force by merely adjusting the motor current like in the device described in JP2011-183513A.
On the other hand, a device which detects the gripping force of the fingers and controls the electric motor in accordance with the detected results is known. For example, the device which is described in Japanese Unexamined Patent Publication No. 2011-194523A (JP2011-194523A) includes an elastic member comprised of rubber or plastic foam between a linear motion member moving along a shaft which is driven to rotate by an electric motor and a coupling member coupled to a gripping member to calculate the gripping force from the amount of displacement of the elastic member.
However, the elastic member described in JP2011-194523A easily changes in modulus of elasticity by the temperature. Further, there is a hysteresis property between the elongation and contraction. Further, the material properties easily change along with aging and invasion of lubricating oil or moisture is liable to cause swelling. For this reason, it is difficult to precisely and stably detect the gripping force.